Make Window::interactiveMoveOffset() proportional
Window::interactiveMoveOffset() stores the move offset in pixels, but it is somewhat annoying to deal with when the window size changes, for example when the window is unmaximized.
This change makes Window::interactiveMoveOffset() store a ratio where the move offset can be found. This simplifies the code a bit and fixes the cursor jumping to the topleft window corner. Although there are other glitches.